208 THOMAS C. CHAMBERLIN AND ROLLIN T. CHAMBERLIN 



and to the wash-products immediately derived from it through 

 glacio-fluvial assortment. 1 The graving of a glacier's bed by 

 rock fragments set in its base or sides may be cited as specific 

 evidence of an essentially rigid hold on the graving tools, and of 

 an internally rigid, rather than fluent, motion of the mass holding 

 the tools. The glacial grindings that are borne out with the 

 subglacial waters and give milkiness to glacial streams seem to 

 us irrefutable evidence of effective rasping and grooving of the 

 rigid type, not of simple viscous overcreep. The very marked 

 contrast between the turbid waters that flow from beneath glaciers 

 and the relatively clear waters that flow down adjacent ungla- 

 ciated valleys is very impressive and spectacular evidence of the 

 superior erosive powers of glaciers. 



Closely allied to this lesson from grindings in transit is a less 

 obtrusive one drawn from the contrast in the points where coarser 

 matter which only strong transporting agents can handle is con- 

 centrated respectively in glaciated and in non-glaciated valleys 

 in regions of the same general type. The upper parts of non- 

 glaciated mountain valleys in cold regions are usually burdened 

 with heavy talus and large loose masses which the drainage is 

 unable to carry away, while the glaciated parts of similar valleys 

 are usually well scoured out and the moutonneed sides and bottoms 

 of U-shaped troughs take the place of the craggy outliers of V- 

 shaped trenches in unglaciated valleys. But in the lower portions 

 of the glaciated valleys below the reach of recent glacial action, 

 aggradation very generally prevails, while in similar non-glaciated 

 valleys degradation generally prevails, if not absolutely, at least 

 relatively. Students of Alpine regions will recall multitudes 

 of illustrations. A similar lesson is even more impressively 

 enforced on the borders of the late Pleistocene glacial areas. In 

 strong contrast to the state of the valleys of the adjacent driftless 

 regions, the great glacio-fluvial valley trains with their thick 

 heads next to the ice border, as well as the frontal aprons, show 

 very conclusively the overladen condition of the glacial waters and 

 their marked incompetency to fully carry away their burdens. 



1 "Hillocks of Angular Gravel and Disturbed Stratification," Am. Jour. Sci., 

 XXVII (May, 1884), 378-90. 



